The present invention relates to a frequency stabilised semi-conductor laser, which may be used in an interferometer system, for example, to measure displacement.
It is known to stabilise the frequency of such a laser by directing the output beam of the laser onto an optical component which interacts with the beam. A parameter describing the interaction, and which is dependent upon the frequency of the incoming beam is monitored to provide an error signal that may be used to stabilise the diode frequency. One example of the use of this technique employs a Fabry-Perot interferometer, or etalon. The intensity of the beam transmitted through the etalon depends upon the frequency of the incoming beam, and thus monitoring this parameter provides a suitable error signal. In a further example the laser beam is directed at an angle onto a reflective grating. The angle at which one of the resultant beams is reflected and diffracted from the grating is monitored (since this parameter is dependent upon the frequency of the incident beam) to provide an error signal.
A problem with such a method of stabilisation is that the parameters of interaction which are sensitive to frequency are also dependent, inter alia, upon the angular orientation of the optical device with respect to the incoming beam. A shift in relative angular displacement of the beam and device will therefore also alter the error signal, even if no shift in frequency has occurred. One aspect of the present invention seeks to overcome this problem by providing an apparatus for stabilising the output frequency of a semi-conductor laser comprising: an optical device situated in the laser beam path, and which interacts with the beam, wherein a parameter describing the interaction varies with (a) incident beam frequency, and (b) angle of incidence between the incident beam and the device; means for producing from the incident beam at least two non-parallel beams each of which is incident upon at least one optical detector, the outputs of the detectors being processed to produce an error signal for controlling operation of the laser in order to stabilise its output frequency, the error signal being sensitive to changes in frequency, and insensitive to changes in angle of incidence.
The optical device may, for example, be an etalon, a grating or an absorption cell, or any other device sensitive to frequency and incident angle.